1. Field of the Invention
This invention pertains to radio frequency (RF) antennas, and in particular to RF antennas adapted for short bursts of signal transmission, where a short burst is characterized by a discrete signal with no residual antenna resonance.
2. Prior Art
Since the inception of electromagnetic theory and the discovery of radio frequency transmission, antenna design has been an integral part of virtually every telemetry application. Countless books have been written exploring various antenna design factors such as geometry of the active or conductive element, physical dimensions, material selection, electrical coupling configurations, multi-array design, and electromagnetic waveform characteristics such as transmission wavelength, transmission efficiency, transmission waveform reflection, etc. Technology has advanced to provide unique antenna design for applications ranging from general broadcast of RF signals for public use to weapon systems of highly complex nature.
Two particular areas of prior art have specific relevance to the present invention. First, U.S. Pat. Nos. 4,028,707 and 4,062,010 illustrate various antenna structures consisting of wire and metal conductors which are appropriately sized for antenna operation with ground penetrating radar. Second, U.S. Pat. Nos. 3,404,403 and 3,719,829 describe the use of a plasma column formed in air by laser radiation as the antenna transmission element.
In its most common form, the antenna represents a conducting wire which is sized to emit radiation at one or more selected frequencies. To maximize effective radiation of such energy, the antenna is adjusted in length to correspond to a resonating multiplier of the wavelength of frequency to be transmitted. Accordingly, typical antenna configurations will be represented by quarter, half and full wavelengths of the desired frequency. Effective radiation means that the signal is transmitted efficiently. Efficient transfer of RF energy is achieved when the maximum amount of signal strength sent to the antenna is expended into the propagated wave, and not wasted in antenna reflection. This efficient transfer occurs when the antenna is an appreciable fraction of transmitted frequency wavelength. The antenna will then resonate with RF radiation at some multiple of the length of the antenna.
Although this essential resonating property is fundamental to the construction of an effective antenna, it also creates a dichotomy where a short burst of RF radiation is desired. For example, in many instances, a short pulse of emitted RF radiation is desired in a discrete packet having sharply defined beginning and ending points. One such application is in radar transmissions where reflections of the radiation are of primary interest. These reflections (backscatter) occur as the electromagnetic radiation passes through materials of differing dielectric constant. It is often desirable that these reflections provide detectable properties that whose interpretation can identify the object of interest (airplane, missile, etc.). The predictability of the reflected signal is in part dependent upon the uniform nature of emitted signals at the antenna and interference by secondary reflections with the returning signal.
The dominant use of radar has been within the aerospace industry. One reason that radar has generally been focused in this application is because an atmosphere environment is of uniform continuity and provides an ideal transmission medium. Therefore, an airborne object is easily distinguished because it is generally an isolated structure that provides an uncluttered reflection. It is therefore easy to identify an airborne object by its electromagnetic reflection.
However, an area of increasing interest and importance is ground penetrating radar. The ability to map what is beneath the surface of the earth or under debris has become necessary for a variety of reasons. For example, locating the precise position of underground pipes and cables can be accomplished without wasting time digging, and with minimal disturbance of soil. However, in this instance, the variety of materials (rocks, sand, soil, vegetation and debris) in the transmission medium with varying dielectric constants creates an array of RF reflections that resemble background noise and clutter. In an effort to minimize the amount of background reflection, the common practice has been to emit a small burst of RF energy, and then evaluate the reflected signal based on this short burst. In this manner, the reflections are limited to short pulses, rather than a repeating wave front. Backscatter is therefore clearer if (i) there is no interference with new signals from the transmission source, and (ii) multiple reflections between target objects are held to a minimum. Thus, it is desirable to terminate all transmission signals before a new signal is sent.
U.S. Pat. Nos. 4,028,707 and 4,062,010 by Young et. al. illustrate two similar approaches for generating and detecting wave pulses within a ground radar application. As will be noted, substantial emphasis is placed on techniques for forming the wave pulse, including design considerations for the transmitting antenna. Numerous configurations for improving the shape of the emitted pulse have been conceived during the twenty-five years since issuance of the respective patents by Young et al.
Despite the need and ongoing interest in improving antennas capable of generating a discrete pulse transmission, a recurring problem is the resonating nature of the antenna. FIG. 1 illustrates a one cycle signal 10 such as might be broadcast from a conventional antenna. At time T.sub.1 the RF transmission coupled to the antenna is cut off; however, a residual signal 11 continues to oscillate over the trailing period despite termination of RF transmission energy to the antenna. When applied within a ground radar system, this trailing resonance signal 11 causes numerous reflections that create a complex array of unmanageable backscatter signals that generally resemble clutter. Obviously, it would be much preferred to have the one cycle pulse cut off instantly, leaving only reflections of the original signal 10, with no residual antenna resonance oscillations to create confusing reflections.